Introduction

Most of the time, the immune system works like a team of security guards protecting the body against outside threats. But sometimes, it can get its wires crossed and start attacking the body itself, like in Cerebral Folate Deficiency (CFD).¹ In this condition, the immune system can produce autoantibodies that attack the machinery responsible for transporting a nutrient called folate (vitamin B9) into the brain.² Conveniently, these autoantibodies can be easily detected with certain tests, which can help clinicians to diagnose patients with suspected CFD and design an effective treatment plan.³

In this article, we will cover what CFD is, explore what autoantibodies are, and their role in CFD. We will also discuss how testing for autoantibodies in CFD works, how to interpret the test results, and how this can inform treatment choice.

Understanding CFD

What is CFD?

CFD is a rare but treatable condition in young infants in which, although folate levels in the blood appear normal, there is a shortage of folate in the brain.² Folate is essential for brain growth and repair processes; a lack of which can have many downstream consequences.⁴

What are the symptoms of CFD?

Onset of CFD usually begins at four months post birth.³ Symptoms can include:⁵

• Irritability
• Insomnia
• Slow head growth
• Neurodevelopmental delay or regression – slow progress towards or loss of abilities like walking and talking
• Ataxia – muscle control issues
• Dyskinesias – involuntary jerking movements
• Hypotonia (weak muscles) or spasticity (stiff muscles)
• Hyperreflexia (exaggerated reflexes, often in the legs and feet)
• Seizures – less common
• Vision problems – in children aged three or older³

What causes CFD?

CFD occurs when folate receptor alpha (FRα) - the protein that folate binds to so it can be moved from the blood into the brain - stops working properly.³ This can be due to mutations (changes) in the DNA used to make FRα, or more commonly because of autoantibodies that block its function (explained in more depth below).²

Autoantibodies and Their Role in CFD

What are antibodies?

Before discussing autoantibodies, we must first address antibodies in general. Antibodies are proteins made by the immune system that defend the body against pathogens like viruses or bacteria.^6,7 They recognise proteins commonly found on the surface of pathogens, called antigens, and stick to them like glue to form an antigen-antibody complex.⁸ This complex acts like a “red flag” that tells other immune cells to destroy the pathogen before it can cause further harm.⁷

Antibodies are also highly selective – each type has a specific shape, meaning they can only bind to a specific antigen.⁹ This allows the immune system to produce an efficient and targeted response against invaders. However, sometimes this system struggles to distinguish between these troublemakers and the good guys…

What are autoantibodies?

Autoantibodies are antibodies that bind to specific antigens native to the body, rather than antigens on invading pathogens.¹⁰ Essentially, this means that the immune system starts to attack healthy, naturally occurring proteins in the body as if they are harmful. It is not always clear what provokes autoantibody production, but genetics and inflammation are both thought to play a role.¹⁰

When autoantibodies latch onto a protein, they can block, damage or even destroy it, which can interfere with many processes needed to keep the body healthy.¹¹

FRα Autoantibodies in CFD

In CFD, the immune system produces autoantibodies against FRα, disrupting folate transport into the brain.² This prevents brain cells from getting the folate they need to function properly, which can cause many symptoms of CFD.

FRα autoantibodies come in two types:¹²

• Blocking
  • Attach to FRα in the same place that folate usually does
  • This prevents folate from binding, as its “seat” is occupied
• Binding
  • Attach to FRα in a different place to the site folate usually binds
  • This can cause inflammation of and damage to the folate binding site on FRα, making it difficult for folate to bind, like chewing gum in a lock preventing the key from fitting properly

Though patients with suspected CFD are tested for both types,³ blocking FRα autoantibodies are more consistently reported in CFD patients,¹³ while binding FRα autoantibodies are more linked to autism spectrum disorder.¹⁴

Getting Tested for FRα Autoantibodies

When is it necessary?

An FRα autoantibody test is one part of the process in diagnosing CFD. First, clinicians will take a full medical history and discuss the patient’s symptoms. Crucially, they will also measure the levels of folate in the blood and cerebrospinal fluid (CSF) – the fluid that surrounds the brain and spinal cord.² If folate levels are low in both samples, this would indicate a systemic folate deficiency. However, if folate levels are only low in the CSF and blood folate levels appear normal, this suggests the patient has CFD.¹⁵ At this point, an FRα autoantibody test can be performed to help determine if FRα autoantibodies are the cause of this case of CFD.³

How does the test work?

First, a blood sample is taken from the patient with suspected CFD.¹² In infants, this may be a finger or heel prick, but a blood draw from a vein in the arm is preferred.¹⁶

The blood sample is then taken to a laboratory and tested for the blocking and binding of FRα autoantibodies. Although the methods used for the two tests differ slightly, both reveal how much of each antibody type is present in the sample.³

Interpreting the results

High levels of either type of FRα autoantibody in the blood would indicate a positive result.

Low levels of either type of FRα autoantibody in the blood would indicate a negative result.

If the results are unclear or borderline, repeating the test later may be necessary.

Clinical Relevance of Autoantibody Test Results

Allows early detection

Infants are notoriously difficult patients. Most adults can explain to a doctor what the problem is, but it is much harder to ask a baby what is wrong and get a coherent answer. Even older children with CFD, especially those with speech delay or regression, may struggle to describe the symptoms they are experiencing. While signs of CFD often begin to emerge around four-to-six months of age, the full clinical picture may not be obvious until after 30 months.³ This can mean months of unnecessary tests, waiting and distress for the family. Moreover, CFD patients treated at a younger age tend to have better outcomes,² so catching it early is crucial.

Performing an FRα autoantibody test allows clinicians to get ahead. This minimally invasive test can help diagnose CFD as soon as it suspected, allowing treatment to start as early as possible.²

Informs treatment choice

A positive test result for FRα autoantibodies is a clear sign to begin folinic acid therapy.²

Folinic acid (also called leucovorin) is the natural, biologically active form of folate.¹⁷ Unlike other forms of folate which rely on FRα to access the brain, folinic acid can use a different transporter - the reduced folate carrier.¹² When given in high enough doses, folinic acid can use this alternative route, bypassing the FRα pathway entirely, allowing a steady supply of folate to the brain even in the presence of blocking FRα autoantibodies.³

Folinic acid therapy has been linked to massive improvements in CFD symptoms in patients with blocking FRα autoantibodies, and even complete recovery in some cases.^{13, 18} This shows how invaluable FRα autoantibody testing is – it identifies the likely cause of CFD and points directly to a targeted, highly effective treatment.

Guides further testing

A negative test result for FRα autoantibodies may point to an alternative cause of CFD symptoms.

In this case, genetic testing may be necessary to determine if a mutation in genes that regulate folate metabolism, like FOLR1,¹⁹ might be the culprit.

If these tests are also negative, this may suggest that CFD symptoms are secondary to another disorder, such as Rett Syndrome or Kearns-Sayre Syndrome.²

Therefore, while a negative result may initially seem disappointing, it brings clinicians one step closer to identifying the cause of CFD symptoms, and most importantly, treating them.

Monitoring changes and recovery

FRα autoantibody levels are known to change with time, such that a positive result could become negative, or vice versa.² Don’t worry if this happens – both gradual and more rapid changes in autoantibody levels are common in infants with CFD.³ As a result, clinicians will often repeat these tests across multiple weeks or months to get a clearer picture of the condition to determine what next steps are required.

Additionally, FRα autoantibody levels can fluctuate over the course of specific treatments for CFD, like the introduction of a milk-free diet.²⁰ Tracking these changes in autoantibody levels throughout treatment can help clinicians monitor how well the treatment is working, and if it should be changed, intensified, or stopped altogether.

Summary

CFD is a condition characterised by low folate in the brain, which can cause developmental delay and regression, seizures and other symptoms in infants.

Autoantibodies are proteins made by the immune system that attack the body’s own tissues. Autoantibodies that attack FRα – the protein that moves folate from the blood into the brain – are one of the primary causes of CFD.

Testing a patient for FRα autoantibodies can help clinicians to diagnose CFD early, guide treatment and monitor the condition over time. A positive result indicates that folinic acid therapy might be beneficial, while a negative result suggests further testing is necessary to determine the cause of CFD symptoms, and consequently what treatment will work best.

Overall, FRα autoantibody testing is crucial for timely diagnosis, targeted treatment, and effective long-term care of patients with CFD.